The present invention relates to a method for simultaneously applying a plurality of coating solutions onto a moving support, thereby to form multiple layers of coating solutions thereon. More particularly, the invention pertains to a method for manufacturing a multi-layer magnetic recording medium by simultaneous coating.
Recently, magnetic recording media have been improved in recording density and decreased in thickness. In association with this technical advancement, the current tendency is to use a multi-layer magnetic recording medium instead of a single-layer magnetic recording medium.
In order to improve the adhesion of a magnetic layer or back layer and its support, an undercoating layer is provided on the support. That is, in order to manufacture magnetic recording media, it is necessary to form two or more layers on one support.
In manufacturing such a multi-layer magnetic recording medium, coating and drying are carried out after each layer is formed on the support. That is, coating and drying operations must be repeatedly carried out to manufacture a single multi-layer magnetic recording medium. Such a method though is low in productivity and high in the cost of required equipment.
Hence, there has been a strong demand for the provision of a method, hereinafter referred to as "a multi-layer coating method", for forming a plurality of layers on a support in one coating and drying cycle.
Examples of known magnetic recording medium coating methods are roll coating, gravure coating and extrusion-plus-doctor coating methods. In these coating methods, however, it is impossible to simultaneously form a plurality of film layers on one support because of the shear stress which is produced during the coating operation.
On the other hand, a slide coating method using a slot has been disclosed, as one example of a multi-layer coating method, for the manufacture of photographing photosensitive materials. Japanese Published Patent Application No. 12937/1981 discloses a multi-layer coating method for magnetic recording materials which uses a coating apparatus with a slide. However, in the manufacture of magnetic recording media, the coating solutions have a high viscosity and dry quickly being organic solvent dispersion solution which are liable to aggregate, and are hence this method is not suitable for high-speed film layer coating operations.
In addition, a variety of extrusion-type coating methods have been disclosed in the art; however, almost all are limited to forming a single film layer on a support.
Japanese Unexamined Published Patent Application (OPI) No. 109162/1983 discloses a coating method in which an extrusion coating head is pressed against a web (support) is employed to form a plurality of film layers thereon. However, the method is advantageous in the following points: (1) It is necessary to provide a doctor edge for each layer. The edges are liable to wear, and it is rather difficult to set the edges at the proper position. (2) The layers are formed separately, and therefore the stable coating range of each layer is small. Particularly the stable coating range of the lowermost layer (the layer directly above the support) is small.
On the other hand, Japanese Published Patent Application No 8977/1958 and Japanese Patent Application (OPI) No. 111168/1986 disclose a method in which a plurality of coating solutions are allowed to meet together in a hopper (pocket) or slot inlet and are then extruded. These methods are still disadvantageous in the following points: (1) In the case where the slot inlet is divided with a partition board or the like, it is technically difficult to make the gap between the partition board and the slot uniform in the widthwise direction. Accordingly, it is impossible to make the coating thickness uniform in the widthwise direction. (2) When coating solutions differing in viscosity are allowed to flow over a long distance after meeting, the solutions may be mixed in the interface therebetween. Thus, the flow of the solutions becomes unstable.
U.S. Pat. No. 2,901,700 discloses a coating method in which an intermediate edge having a sharp or flat end face is arranged in the slot to apply two coating solutions to the support. In this case, since the intermediate edge is made of an ordinary material, small flaws several to several tens of microns in length are inevitably formed on the end face. As a result, strips are formed on the product. In the case where an intermediate edge having the flat end face is used, depending on the width of the flat end surface, the width of the solution path may change abruptly, as a result of which vortices are formed in the stream of coating solutions, whereby the two solutions are mixed, spoiling the interface therebetween. Hence, the resultant product also has strips on its surface.
The above-described difficulties become more significant as the coating speed increases and the coating thickness is reduced.